The invention relates generally to high-pressure plunger pumps used, for example, in oil field operations. More particularly, the invention relates to plunger packing and stress reduction in plunger pump housings.
Plunger Pump Stress Failure
Engineers typically design high-pressure oil field plunger pumps in two sections; the (proximal) power section and the (distal) fluid section. The power section usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc, The fluid section usually comprises a housing which in turn comprises suction, discharge and cylinder bores, plus plungers, packing, valves, seats, high-pressure seals, etc. FIG. 1 is a cross-sectional schematic view of a typical fluid section showing its connection to a power section by stay rods. A plurality of fluid sections similar to that illustrated in FIG. 1 may be combined, as suggested in the Triplex fluid section design schematically illustrated in FIG. 2.
Each individual bore in a fluid section housing is subject to fatigue due to alternating high and low pressures which occur with each stroke of the plunger cycle. Fluid section housings typically fail due to fatigue cracks in one of the four areas defined by the intersecting suction, plunger and discharge bores as schematically illustrated in FIG. 3.
Among the designs proposed in the past for reducing pump housing fatigue failures in high-pressure fluid sections has been the Y-block housing design. The Y-block design, which is schematically illustrated in FIG. 4, reduces stress concentration in a fluid section housing by increasing the angles of bore intersections above 90xc2x0. In the illustrated example of FIG. 4, the bore intersection angles are approximately 120xc2x0. A more complete cross-sectional view of a Y-block plunger pump fluid section is schematically illustrated in FIG. 5.
Although several variations of the Y-block design have been evaluated, none have become commercially successful for several reasons. One such reason is that mechanics find field maintenance on Y-block fluid sections difficult. For example, replacement of plungers and/or plunger packing is significantly more complicated in Y-block designs than in the earlier designs represented in FIG. 1. In the earlier designs, provision is made to push the plunger distally in the cylinder bore, continuing out through an access port labeled the suction valve/plunger cover in the illustration. This operation, which would leave the plunger packing easily accessible from the proximal end of the cylinder bore, is impossible in a Y-block design.
The Y-block configuration, while reducing stress in a fluid section housing, makes it necessary to remove the plunger from the proximal end of the cylinder bore. But because the proximal end of the cylinder bore is very close to the power section, plungers must be removed in two pieces. And even a two-piece plunger, schematically illustrated in FIG. 5, is itself a maintenance problem. The plunger pieces are often heavy and slippery, the connection between plunger pieces is subject to premature failures, and plunger pieces must be connected and disconnected in a confined space with limited visibility and accessibility. Nevertheless, the plunger pieces must be removed entirely from the cylinder bore in order to change conventional plunger packing.
Plunger Packing
A brief review of plunger packing design will illustrate some of the problems associated with packing and plunger maintenance in Y-block fluid sections. FIG. 6 is an enlarged view of the packing in an earlier (but still currently used) fluid section such as that illustrated in FIG. 1. In FIG. 6, the packing and packing brass are installed in the packing box of the fluid section. Note that packing brass is a term used by field mechanics to describe bearing bronze, where the bronze has the appearance of brass.
In the fluid section portion schematically illustrated in FIG. 6, the packing box is an integral part of the fluid section housing; it may also be a separate unit bolted to the fluid section housing. The packing is retained, tightened and adjusted by turning the gland nut. Removing the gland nut, however, does not allow one to remove the packing rings. Because packing rings must block high-pressure fluid leakage past the plunger, they are typically quite stiff, and they remain substantially inaccessible while the plunger (or any piece of it) remains in the cylinder bore. FIG. 7 schematically illustrates portions of a plunger pump housing and components including a gland nut and plunger parts, with the plunger pressure end within the packing box. Note, however, that the plunger pressure end cannot be rotated for removal until it clears the packing brass. This illustrates the necessity for a two-piece plunger in which the two pieces must be separated as they are individually removed from the cylinder bore.
The necessity for a multi-piece plunger in Y-block fluid section housings has not been eliminated by the recent introduction of packing assemblies such as those called xe2x80x9ccartridge packingxe2x80x9d by UTEX Industries in Houston, Tex. An example of such cartridge packing is schematically illustrated in FIG. 8. Note that removal of the gland nut exposes the packing cartridge housing, which in turn may be fitted with attachment means to allow extraction of the packing cartridge from the packing box (requiring proximal travel of the packing cartridge housing of approximately three to five inches).
This extraction, though, is not practical while a plunger piece lies within the packing box because of the excessive drag of the compressed packing rings on the plunger and packing box walls. Such compression can not be released unless all plunger pieces are removed from the packing box because the packing rings in the above cartridge packing assemblies are pre-compressed when the assemblies are manufactured. Further, any slight misalignment of apparatus used to extract such a cartridge packing assembly tends to cause binding of the (right cylindrical, i.e., not tapered) assembly within the (right cylindrical) bore. Analogous difficulties occur if an attempt is made to replace such a cartridge packing assembly while a plunger or part thereof lies in the packing box area. Hence, even if such cartridge packing assemblies were used in Y-block fluid section housings, multi-piece plungers would preferably be used and field maintenance would be significantly complicated and expensive.
The invention comprises methods and apparatus to reduce or eliminate the above described problems of premature fluid section pump housing fatigue failure and difficult field maintenance related to plungers and/or plunger packing. In a preferred embodiment of the invention, a Y-block plunger pump housing comprises a suction valve bore having a substantially circular cross-section and a first centerline. Bore centerlines are used herein to assist the reader in understanding how each bore in the fluid section pump housing is spatially related to other bores in the pump housing and other fluid section components.
A discharge valve bore intersects said suction valve bore, said discharge valve bore having a substantially circular cross-section and a second centerline, said first centerline preferably being coplanar with and intersecting said second centerline at a reference point, and said first and second centerlines subtending a first obtuse angle.
A cylinder bore intersects said suction valve bore and said discharge valve bore, said cylinder bore having a proximal packing area (relatively nearer the power section) and a distal transition area (relatively more distant from the power section). The packing area has a substantially circular cross-section and a third centerline. The third centerline is coplanar with said first and second centerlines and intersects them at or near said reference point to allow substantially unimpeded fluid flow from said suction bore to said discharge bore under the influence of reciprocating plunger movement in said cylinder bore. Said second and third centerlines subtend a second obtuse angle, and said first and third centerlines subtend a third obtuse angle. Preferred values for the first, second and third obtuse angles, as well as preferred intersections of the first, second and third bore centerlines, are determined primarily by design guidelines which minimize materials and machining costs. Such guidelines are well known to those skilled in the art.
The transition area of the cylinder bore has a distal elongated cross-section substantially perpendicular to said third centerline and with a long axis substantially perpendicular to the plane of said first, second, and third centerlines. Modern computer-aided finite element stress analysis (FEA) was used to study stress concentrations in the fluid section pump housing design of the present invention and to document the advantages of the above elongated cross-section. Past Y-block pump housing designs, on the other hand, experienced premature fatigue-induced cracks due to stress concentrations that could not be predicted without computers and modern FEA software.
Note that FEA reveals that elongation of the distal portion of the cylinder bore transition area as described above is generally beneficial in reducing stress near the intersections of the cylinder bore transition area with the suction and discharge bores. The shape of the elongation, however, may be optimized to obtain the greatest stress reduction. For example, while an elliptical cross-section is beneficial, an oblong cross-section is more beneficial.
The cross-section of an oblong bore consists of two opposing half-circles connected by substantially straight lines, which leaves a substantially flat area between the cylindrical sections of the oblong bore. These substantially straight lines preferably have length between 5% and 95% of the length of radii of the opposing half circles. The unexpected result of incorporating such an oblong bore is that stresses in all areas of the intersecting bores of the present invention are significantly reduced. Note that stresses are reduced in spite of the fact that pump housing material is removed and the fluid section side wall thickness is reduced in the area of the oblong bore, which would ordinarily be expected to increase stress concentrations rather than reduce them.
An explanation of this surprising phenomenon lies in the intersection of the suction and discharge bores with the flat area of the oblong bore, which (FEA analysis shows) disperses stresses along the flat area. Note that the presence of the flat area effectively increases any discrete angles of intersection between the suction and discharge bores and the cylinder bore. Indeed, by tapering the oblong cylinder bore to flare out from proximal to distal, the transition from either the suction or discharge bore to the right cylindrical portion of the cylinder bore can be made nearly smooth. In contrast, earlier (circular) cylinder bores tend to concentrate stresses where they intersect with circular suction and discharge bores, discrete angles of intersection being relatively smaller than in the present invention.
Another preferred embodiment of the present invention relates to a tapered cartridge packing assembly comprising a packing cartridge housing and related components. The packing cartridge housing has a distal end, a proximal end, a longitudinal axis, and a length between said distal and proximal ends. A substantially right cylindrical inner surface of the cartridge housing has a first diameter, and a substantially coaxial right cylindrical outer surface extends distally from said proximal end for a portion of said cartridge housing length. A conically tapered substantially coaxial outer surface extends distally from said distal extent of said right cylindrical outer surface to said cartridge housing distal end, said tapered outer surface tapering distally from said right cylindrical outer surface toward said longitudinal axis. The inner surface has a substantially coaxial cylindrical recess having a second diameter greater than said first diameter and extending from said distal end proximally to an internal anti-extrusion ring stop. The cylindrical recess has a substantially coaxial internal snap ring groove, said groove having a substantially uniform width and a third diameter greater than said second diameter.
There is at least one circumferential seal groove in said right cylindrical outer surface, and an elastomeric seal is fitted within each said circumferential seal groove. A substantially coaxial bearing ring lies within the cylindrical recess; it has an inner diameter slightly less than said first diameter and an outer diameter about equal to said second diameter. The bearing ring contacts said bearing ring stop. A substantially coaxial anti-extrusion ring also lies within the cylindrical recess. The anti-extrusion ring contacts said bearing ring. With an inner diameter slightly less than said first diameter and an outer diameter about equal to said second diameter, the anti-extrusion ring has a close sliding fit against a plunger in the cylinder bore, thereby effectively preventing extrusion of plunger packing proximally.
A substantially coaxial snap ring having a thickness less than said snap ring groove width lies within the snap ring groove. The snap ring has an inner diameter slightly greater than said first diameter and an outer diameter slightly less than said third diameter, said snap ring having a longitudinal sliding fit within said snap ring groove.
A substantially coaxial packing compression ring has an inner diameter slightly greater than said first diameter, an outer diameter slightly less than said second diameter, and a thickness preferably greater than said snap ring groove width reduced by the snap ring thickness. The packing compression ring is positioned between said snap ring and said anti-extrusion ring and contacts said snap ring but is too thick to become lodged in said snap ring groove when the snap ring is in place in the groove.
A substantially coaxial packing ring lies within said cylindrical recess. The packing ring has an inner diameter substantially equal to said first diameter and an outer diameter substantially equal to said second diameter. It has sufficient length to substantially fill said recess between said anti-extrusion ring and said packing compression ring when said snap ring is positioned maximally distally within said snap ring groove. Note that proximally directed longitudinal sliding movement of said snap ring within said snap ring groove causes proximally directed longitudinal sliding movement of said packing compression ring with resultant compression of said packing.
A tapered cartridge packing assembly of the present invention is advanced distally into the tapered recess of the packing area of a cylinder bore of a plunger pump housing of the present invention through distal motion imparted by tuning a threaded gland nut. The gland nut may be separable from the tapered cartridge packing assembly, but in an alternative preferred embodiment (a tapered cartridge packing and gland nut assembly), the gland nut is integral with the proximal end of the packing cartridge housing. Before being advanced distally, the coaxial packing ring is uncompressed, which means that drag on a plunger which may be within the packing area of the cylinder bore is relatively low. When the packing assembly is nearly fully inserted into the packing area (that is, within a distance from the end of its travel equal to the snap ring groove width), the snap ring encounters a coaxial cylindrical boss of the pump housing, the proximal face of which is termed the adjusting ring. Further (distal) advance of the packing assembly after the snap ring contacts the adjusting ring results in relative proximal longitudinal movement of the snap ring in its groove. This proximal longitudinal movement results in compression of the coaxial packing ring with a consequent tightening of the packing around the plunger.
Because of the shallow taper of a distal portion of its outer surface (preferably in the range of 0.5 to 3 degrees) and the circumferential elastomeric seal present on a proximal portion of that surface, a tapered cartridge packing assembly will maintain an effective seal with a plunger pump housing during longitudinal sliding movement less than or equal in magnitude to the snap ring groove width. Thus, as described above, the degree of tightening of packing around a plunger may be adjusted by varying the distance a packing assembly is advanced into a plunger pump housing of the present invention after the snap ring contacts the adjusting ring. Note that during advance and withdrawal of a packing assembly, the tapered portion tends to maintain alignment with a cylinder bore, thus minimizing any tendency to bind.
Note also that distal advance of a tapered packing assembly or tapered packing and gland nut assembly of the present invention is preferably limited by the snap ring or the gland nut shoulder, rather than by the assembly being wedged tightly into the tapered recess of a cylinder bore packing area. These complementary provisions to limit distal advance also act to minimize binding of the assembly in the tapered recess. Thus, withdrawal of a tapered packing assembly should be substantially free of binding while drag due to packing compression is substantially reduced as the assembly is withdrawn and the snap ring becomes free to move distally in its groove to relieve compression of the packing ring. These effects combine to make changing of packing with a plunger in the cylinder bore practical in the field.